Structure of the tannin geraniin based on conventional X-ray data at 295 K and on synchrotron data at 293 and 120 K
Luger P, Weber N, Kashino S, Amakura Y, Yoshida T, Okuda T, Beurskens G, Dauter Z
ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE 54: 687-694 Part 5 OCT 1 1998

Abstract:
Geraniin, C41H28O27.7H(2)O, is the main tannin from Geranium thunbergii Sieb. et Zucc., which is one of the most popular folk medicines and also an official antidiarrheic drug in Japan. Conventional X-ray diffractometer data at 295 K and synchrotron data at 293 and 120 K (at EMBL/DESY Hamburg with a MARRESEARCH imaging plate) were measured. The structure could not be determined with any of the currently distributed direct-methods programs, but was easily solved with DIRDIF using several conformers of the hexahydroxydiphenoyl group as input fragments into a vector-search procedure. A molecular structure was established where a cyclohexenetrione moiety attached to O4 of a central glucose unit was in a hydrated six-membered hemiacetal ring structure, as expected from the NMR study of crystalline geraniin. Owing to the high oligocyclic substitution the glucopyranosyl ring is in the unusual C-1(4) conformation. Refinement of the low-temperature synchrotron data allowed identification of almost all the H atoms, even within the seven water molecules, so that a rather complex system of approximately 30 different hydrogen bonds can be studied in some detail.